Recursive Reduce

reduce.js

var boringArray = [1,2,3,4];
var datesArray = [1878, 1980, 1535, 1879];

var reducer = (a, b) => a + b;
var earliestReducer = (a, b) => {
  if(a < b) {
    return a;
  }
  
  return b;
}

const rReduce = (arr, func, acc) => {
    acc ? acc : arr[0];
  
    if(arr.length === 0) {
        return acc;
    }

    return rReduce(arr.slice(1), func, func(acc, arr[0]))
};

console.log(rReduce(datesArray, earliestReducer));
console.log(rReduce(boringArray, reducer, 0));